A function which is frequently required in technology is the separation of liquid droplets from a gas flow.
The separation devices used for that purpose are frequently so designed that a mostly cylindrical filter insert (a filter cartridge) is fitted in a housing, with the gas flow to be cleaned flowing radially through the filter insert. In that case the flow through the filter insert can be from the peripheral surface inwardly or vice-versa. A typical filter insert includes a media packet (a packet of filter media) which is fixed by securing portions to the cover surfaces.
The filter media used in a filter insert have to perform two main functions:                separation of the entrained liquid by various filter mechanisms, predominantly depth filtration and coalescence, and        continuous discharge of liquid from the filter medium to maintain a low level of saturation of the filter medium and thus a low counterpressure in the filter.        
The usual filter design involves a structure having one or more different filter media, with suitable media properties like fiber diameter, porosity and surface energy. In general coarse separation is disposed upstream of fine separation in the flow direction.
The design of a filter is implemented having regard to the demands in terms of degree of separation, liquid drainage and acceptable counterpressure.
Filter media for fine separation are required in particular for high degrees of separation. Typically the filter media for fine separation are characterised by a small fiber diameter and small pore size.
A disadvantage there is that absorbed liquid, for example oil, is not adequately transported out of the filter medium for fine separation, under usual operating conditions, whereby this involves a high flow resistance in operation.
In order to minimize saturation of the filter media with separated liquid there are endeavors to improve drainage from the filter media (media for coarse separation and for fine separation), in the direction of the gas flow.
AT 512506 B1 shows a filter device for separating droplets from gaseous material flow, wherein at an entry surface of a filter layer a flow barrier is formed, which partly covers the area of the entry surface adjacent to a drainage layer and wherein the flow barrier substantially completely covers the entry face of the drainage layer.
DE 202005004151 U1 describes a filter element, at which a collected liquid can flow over a bottom edge of a spacer fabric through gravitation and can be collected afterwards.
DE 69909284 T2 describes improvements at a coalescence filter. According to the document, the filter comprises a first oil coalescence layer made of a micro fiber material, and a second layer from an oil drainage material, wherein a drainage layer serves to accommodate oil from the coalescence layer and offers a path to the oil to flow out of the filter by gravitation.
US 2010/031940 A1 shows a filter device for filtering blow-by gas, with axial drainage and sealing.
U.S. Pat. No. 8,499,749 B2 discloses a filter for a blow-by gas of a crankcase venting system, in which drainage holes for the separated liquid are provided in the bottom portion of the filter insert. It will be noted however that the disadvantage of this solution that inevitably there is a bypass of gas through the drainage openings. Bypass means here that gas can escape through the drainage openings and thus does not pass through all filter layer portions.